This invention relates generally to gas turbine engines and, more particularly, to start systems for gas turbines of the turbofan type.
The method by which gas turbine engines are started is by rotating the high pressure compressor up to a speed sufficient to provide air, under pressure, to support combustion in the combustion chamber. After the engine is fired, the starter must assist the engine until it has reached the self-accelerating speed, with the torque required being in excess of the torque required to overcome rotor inertia, engine accessory loads, the friction loads of the engine, and the extracted loads of aircraft accessories. Various power sources are used to start a gas turbine engine, among which are the use of electricity, gas, air and hydraulic pressure. Whichever method is used, it must be capable of developing a very high amount of power in a short time and transmitting it to the engine rotating assembly in a manner which provides a smooth acceleration from rest, up to a speed at which the gas flow provides sufficient power to the engine turbine to enable it to take over. This requirement is easily met by many different types of on-ground power supply systems or from power systems carried aboard the aircraft. If a flameout occurs to an engine during flight, the supply of thermal energy to the turbines will discontinue and the rotational speed of the compressor spool will accordingly decrease considerably; however, the engine will continue to rotate due to the flow of air through the compressor, a phenomenon which is commonly referred to as windmilling. In a turbojet engine, there is a large volume of air which passes through the compressor following a flameout, and the windmill speed of the core engine is sufficient for an in-flight start.
In the case of a turbofan engine, however, wherein a good portion of the air which enters the inlet of the engine passes around the engine core, the high pressure compressor rotor receives a smaller portion of the available ram energy and therefore does not attain as high a windmill speed as in turbojet engine. This is particularly true of a turbofan of the mixed-flow type where a common nozzle provides additional restriction to core airflow and lowers windmill speed. If the windmill speed of the core is not sufficient, then an air start cannot be obtained without some kind of starter assist. Since the ability of an engine to relight varies with altitude and forward speed of the aircraft, a starter assist may not be required over the entire flight envelope of an aircraft, but only over a portion thereof, such as, for example, during periods of low-speed flight.
One method by which a starter assist is provided for air starts is that of an auxiliary power unit (A. P. U.) wherein a gas turbine located aboard the aircraft provides shaft power to the core by way of a gearbox. After the engine is started and a predetermined engine speed is attained, a control valve is automatically closed and automatically disengages the drive mechanism. Another method employed is that of cartridge starting, wherein the starter motor is basically a small impulse-type turbine which is driven by high velocity gases from a burning cartridge. The power output of the turbine is passed through a reduction gear and an automatic disconnect mechanism to rotate the engine. Another method employed is that of the combustor air starter wherein the starter unit has a small combustion chamber into which high pressure air from an aircraft-mounted storage bottle along with atomized fuel are introduced and ignited to generate resultant gases which are directed onto the air-starter turbine.
Whatever method is used, auxiliary torque sources which need start-up or which can only be used once per flight are restrictive in their use.
It is therefore an object of the present invention to provide for a turbofan engine an improved starter-assist system which is always ready for quick and reliable application.
Another object of this invention is to provide a means by which a turbofan engine can be air started over a much larger portion of the flight envelope.
Still another object of this invention is the provision in a turbo-fan engine for an air-start assist system which is relatively light in weight, effective in use, and simple in operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.